Mean chiasma frequencies and the amounts of between-bivalent chiasma variation were computed for pollen mother cells (p.m.c.) and embryo-sac mother cells (e.m.c.) of five distinct inbred rye lines. Despite the considerable differences for both these metrics shown by the different lines, very little variation was seen between p.m.c. and e.m.c. belonging to the same lines. It is concluded that chiasma formation in p.m.c. and e.m.c. of rye is governed and regulated by a single controlling system of genes and that variation in this genetic system is expressed identically in the two sexes.

Gene and genotypic frequencies for a deleterious mutant in mutation selection balance are derived for an infinite population undergoing partial self-fertilization. These provide formulations of mean survival and the mutational load. Obtained also are the average number of mutant genes and affected individuals stemming from a single mutant.

As a concomitant effect on frequencies at a neutral locus the mutational load is distributed disproportionately among the neutral genotypes. For partially recessive mutant genes on the 1, 1-sh, 1-s scale, the effect is to increase the frequency of the heterozygote and to decrease the frequencies of homozygotes at the neutral locus relative to the frequencies expected with complete neutrality. This apparent overdominance at the neutral locus has been shown to be connected with identity disequilibrium rather than linkage disequilibrium. It increases generally as s and h decrease, and as the proportion of self-fertilization and the degree of linkage increase. The apparent overdominance with complete linkage is generally less than double that for free recombination. For partially dominant mutant genes, h ≥ ½, the effects on the frequencies of heterozygote and homozygotes at the neutral locus are reversed.

A selective procedure is described for the isolation of deletions in phage T4. This is based on the properties of partial diploids which possess a tandem duplication covering the rII region. The starting-point of the procedure is a particular s1231/3157 diploid which has a high segregation frequency and so possesses a long duplication. The replacement in this diploid of the large deletion 3157 by the small one 196 further increases the length of this duplication and removes the terminal redundancy of the resulting phage which are then non-viable. New compensating deletions which restore the terminal redundancy and thus the viability to these diploids are thereby selected. Nine new independent T4 deletions have been isolated by this procedure. The length of three has been estimated by terminal redundancy measurements and each found to be appreciably longer than the rII region. While developing the isolation procedure a key diploid intermediate was identified with the property that is was completely stable; that is, unlike typical diploids which continually generate haploid segregants during phage multiplications, this diploid strain produced no viable haploid segregants. This unexpected finding led us to examine the general problem of how tandem duplications can be stabilized. The solution we propose here is a structural one involving new stabilizing deletions which penetrate into one arm of the duplication removing certain essential genes. These stabilizing deletions are located in such a way that all haploid segregants formed by recombination are necessarily non-viable. Further investigations of the original stable diploid, and others isolated subsequently, validated this model for these phage diploids. It also led to the recognition of several novel genetic structures involving repeated DNA sequences which, together with the concept of the stabilizing deletions, could be of general significance.

Six bacteriophages have been used in the classification of 19 plasmids (antibiotic resistance-mediating R factors and FP sex factors which promote host chromosome transfer) of P. aeruginosa isolated in different geographical regions. On the basis of phage-plating responses on isogenic strains of bacteria differing only in the plasmids carried, five groups of plasmids were distinguishable. In general the groups could be correlated with their geographical origin although differences between plasmids from the same region were found. The unique phage-plating responses were also useful in establishing the possible identity of plasmids isolated from the same original strain and given different designations by independent investigators. The classification of the plasmids derived here on the basis of phage-plating responses could be correlated with classifications based upon other phenotypic characteristics described elsewhere. The nature of inhibition of plating of phages B39 and G101 by R18–1 and R18–3 respectively was shown to be due to interference with some aspect of intra-cellular phage replication rather than to plasmid-mediated restriction.

A method is described for detecting centromere linkage in Saccharomyces cerevisiae using a simple monohybrid for the locus in question. The method utilizes the two-spored asci produced by a normal homothallic diploid and depends upon a selective elimination of specific chromatids during spore formation. The pattern of selective elimination was not detected in a heterothallic strain but was found to be a dominant trait in an inter-strain cross. It was shown that gross chromosome aberrations are probably not responsible for the two-spored asci produced by either the heterothallic or homothallic strains.

Mutants of Pseudomonas aeruginosa, which differed in amide growth phenotype from the wild-type strain, were subjected to genetic analysis using the generalized transducing phage F116. The map order of some mutational sites was determined by 3-factor crosses in which a mutation in the linked regulator gene amiR was used as the outside marker to determine the relative order of mutations in the amidase structural gene amiE. Acetamide-positive transductants were recovered in crosses between amidase-negative strains and strains PhB3(PAC377), V2(PAC353) and V5(PAC356) producing mutant amidases which hydrolyse phenylacetamide and valeramide but not acetamide. Some recombinants carried the mutation amiE16 determining the properties of the mutant B amidase produced by strain B6(PAC351) from which both PhB and V class mutants were derived, while other recombinants produced A amidase determined by the wild-type amiE gene.

In the preceding paper (Van de Vate & Symonds, 1974) a deletion, sdl, has been reported that stabilizes an rII diploid by penetrating into the region of the duplication from the right-hand side (if we take the rIIB gene located to the right of rIIA). Here we wish to report on a different stabilizing deletion, sd2. Evidence will be presented in favour of penetration of the duplication from the left-hand side. The right-hand terminus of the deletion is located within the rIIA gene.

A technique is reported for crossing strains of Aspergillus nidulans which produces hybrid cleistothecia in 7 days. The method depends on the partial inhibition of cleistothecium formation of auxotrophic mutants by growth-factor limitation. Cleistothecia form at the junction of two strains inoculated on to complete medium. The method has a number of advantages over standard techniques.

A least-squares diallel analysis of the offspring of crosses of seven inbred lines of house mice was performed for the mean widths of the second (M2) and third (M3) mandibular molars. Significant differences among lines, as well as between inbreds and hybrids (positive heterosis) were found for these characters. Estimates of the heritabilities (from general combining ability variances) were high for both teeth as were estimates of maternal effects. Specific combining ability and reciprocal effects were significant only for the M3. Additionally, X-irradiation reduced the mean number of individuals per litter, and thereby indirectly influenced the widths of both molars.

Wild-type cells of Aspergillus nidulans have undetectable NADL-glutamate dehydrogenase activity when utilizing glucose and high levels of NAD L-glutamate dehydrogenase when utilizing certain amino acids as sole carbon sources.

A mutant, designated gdhCl, has appreciable NAD-GDH activity when utilizing glucose as a carbon source. The gdhC1 mutation is semi-dominant and is located in linkage group III.

The semi-dominant X-linked gene tabby (Ta) in the mouse, and one of its recessive autosomal mimics, downless (dl) each produces a mutant syndrome that includes absence of hairs on the tail due to failure of tail hair follicle initiation. However, whereas downless tails failed to produce hair follicles in culture on the chick chorioallantoic membrane, which is in keeping with the adult phenotype of both downless and tabby mice, tabby tails produced follicles at about 40% of the control level. Furthermore, in contrast to previous findings for downless, the culture of mixed genotype epidermis-dermis combinations provided no evidence of a primary epidermal effect in tabby.

Pseudomonas arvilla mt-2 (ATCC 23073) has been shown to harbour a transmissible plasmid which codes for the degradation of benzoate and m-toluate. Plasmid-borne genetic information codes for the conversion of these compounds to catechol then the assimilation of catechol via the meta cleavage pathway.

A new method for obtaining mitochondrial mutants of Paramecium aurelia is described. Sensitive organisms were first placed in medium containing a low concentration (0·02 mg/ml) of erythromycin (ERY), insufficient to prevent fission. The paramecia were then transferred through a graded series of increasing ERY concentrations, with a period of growth at each concentration. A high yield of ERY-resistant mutants was obtained.

Continuous growth in 0·02 mg/ml ERY also resulted in the accumulation of a spectrum of mutants resistant to various concentrations of ERY. Many of these mutants are also resistant to mikamycin. On returning the resistant paramecia to media lacking ERY, some clones remained resistant whilst others reverted to sensitivity.